Unit 2 DC Power Units
Unit 3 Performance of Rectifiers Unit 4 Filter Circuits
UNIT 1 POWER SOURCES
CONTENTS 1.0 Introduction 2.0 Objectives 3.0 Main Content
3.1 Steams as an Energy Source 3.2 Fossil Fuels and Nuclear fuel
3.2.1 Fossil Fuels
3.2.2 Oil and Natural Gas 3.2.3 Coal
3.3 Solar Energy 3.4 Windmills 4.0 Conclusion 5.0 Summary
6.0 Tutor-Marked Assignment 7.0 References/Further Reading
1.0 INTRODUCTION
Power supply is a supply of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies.
A power supply may include a power distribution system as well as primary or secondary sources of energy such as:
• Conversion of one form of electrical power to another desired form and voltage, typically involving converting AC line voltage to a well-regulated lower-voltage DC for electronic devices. Low voltage, low power DC power supply units are commonly integrated with the devices they supply, such as computers and household electronics.
• Batteries
• Chemical fuel cells and other forms of energy storage systems
• Generators or alternators
Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.
A regulated power supply or stabilized power supply is one that includes circuitry to tightly control the output voltage and/or current to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply’s output, or any reasonable voltage variation at the power supply's input.
2.0 OBJECTIVES
At the end of this unit, you should be able to:
• know what is meant by power supply
• know the major sources of power
• understand the environmental considerations of the sources of power.
3.0 MAIN CONTENT
POWER SOURCES - Sources of Electrical Energy
Electrical energy occurs naturally, but seldom in forms that can be used.
For example, although the energy dissipated as lightning exceeds the world's demand for electricity by a large factor, lightning has not been put to practical use because of its unpredictability and other problems.
Generally, practical electric-power-generating systems convert the mechanical energy of moving parts into electrical energy (e.g.
generator). While systems that operate without a mechanical step do exist, they are at present either excessively inefficient or expensive because of a dependence on elaborate technology. While some electric plants derive mechanical energy from moving water (hydroelectric power), the vast majority derive it from heat engines in which the working substance is steam. The steam is generated with heat from combustion of fossil fuels or from nuclear fission. These energy resources fall into two main categories, often called renewable and non- renewable energy resources. Each of these resources can be used as a source to generate electricity, which is a very useful way of transferring energy from one place to another such as to the home or to industry.
3.1 Steams as an Energy Source
The conversion of mechanical energy to electrical energy can be accomplished with an efficiency of about 80%. In a hydroelectric plant, the losses occur in the turbines, bearings, penstocks, and generators. The basic limitations of thermodynamics fix the maximum efficiency obtainable in converting heat to electrical energy. The necessity of limiting the temperature to safe levels also helps to keep the efficiency down to about 41% for a fossil-fuel plant. Most nuclear plants use low- pressure, low-temperature steam operation, and have an even lower efficiency of about 30%. Nuclear plants have been able to achieve efficiency up to 40% with liquid-metal cooling. It is thought that by using magneto hydrodynamic “topping” generators in conjunction with normal steam turbines, the efficiency of conventional plants can be raised to close to 50%. These devices remove the restrictions imposed by the blade structure of turbines by using the steam or gasses produced by combustion as the working fluid.
Environmental Concerns
The heat generated by an electric-power plant that is not ultimately converted into electrical energy is called waste heat. The environmental impact of this waste is potentially catastrophic, especially when, as is often the case, the heat is absorbed by streams or other bodies of water.
Cooling towers help to dispose waste heat into the atmosphere.
Associated with nuclear plants, in addition to the problem of waste heat, are difficulties attending the disposal and confinement of reaction products that remain dangerously radioactive for many thousands of years and the adjustment of such plants to variable demands for power.
Public concern about such issues—fueled in part by the accidents at the Three Mile Island nuclear plant in Harrisburg Pennsylvania in 1979, and the nuclear plant explosion in the Soviet Union at Chernobyl in 1986—
forced the U.S. government to introduce extensive safety regulations for nuclear plants. Partly because of those regulations, nuclear plants are proving to be uneconomical. Several are being shut down and replaced by conventionally fueled plants.
3.2
Fossil Fuels and Nuclear Fuel
Fuel cells develop electricity by direct conversion of hydrogen, hydrocarbons, alcohol, or other fuels, with an efficiency of 50% to 60%.
Although they have been used to produce electric power in space vehicles and some terrestrial locations, several problems have kept them from being widely used. Most important, the catalyst, which is an important component of a fuel cell, especially one that is operating at around room temperature, is very expensive. Controlled nuclear fusion
steam in generating plants; however, many problems surround its development, and no appreciable contribution is expected from this source in the near future.
3.2.1 Fossil Fuels
Sources of electricity include fossil fuels are found within the rocks of the Earth's surface. They are called fossil fuels because they are thought to have been formed many millions of years ago by geological processes acting on dead animals and plants, just like fossils.
Coal, oil and natural gas are fossil fuels. Because they took millions of years to form, once they are used up they cannot be replaced.
3.2.2 Oil and Natural Gas
Sources of electricity include oil and gases are chemicals made from molecules containing just carbon and hydrogen. All living things are made of complex molecules of long strings of carbon atoms. Connected to these carbon atoms are others such as hydrogen and oxygen. A simple molecule, called methane (CH4), is the main component of natural gas.
Crude oil (oil obtained from the ground) is a sticky, gooey black stuff. It contains many different molecules, but all are made of carbon and hydrogen atoms.
Advantages
These sources of energy are relatively cheap and most are easy to get and can be used to generate electricity.
Disadvantages
When these fuels are burned they produce the gas carbon dioxide, which is a greenhouse gas and is a major contributor to global warming.
Transporting oil around the world can produce oil slicks, pollute beaches and harm wildlife.
3.2.3 Coal
Sources of electricity can include coal, which mainly consists of carbon atoms that come from plant material from ancient swamp forests. It is a black solid that is reasonably soft. You can scratch it with a fingernail. It is not as soft as charcoal, however, and is quite strong. It can be carved into shapes. There are different types of coal. Some contain impurities such as sulphur that pollute the atmosphere further when they burn, contributing to acid rain.
Advantages
Coal is relatively cheap, with large deposits left that are reasonably easy to obtain, some coal being close to the surface. It is relatively easy to transport because it is a solid.
Disadvantages
a. Some sources of coal are deep below the ground, as in the UK.
They can be difficult, costly and dangerous to mine.
b. Burning coal without first purifying it contributes to global warming, as well as to the production of smog (smoke and fog), which is harmful to health. It is a finite resource and will eventually run out.
3.3 Solar Energy
Solar energy has been recognized as a feasible alternative. It has been suggested that efficient collection of the solar energy incident on 14% of the western desert areas of the United States would provide enough electricity to satisfy current demands. Two main solar processes could be used. Photovoltaic cells (see solar cell) convert sunlight directly into electrical energy. Another method would use special coatings that absorb sunlight readily and emit infrared radiation slowly, making it possible to heat fluids to 1,000°F (540°C) by solar radiation. The heat in turn can be converted to electricity. Some of this heat would be stored to allow operation at night and during periods of heavy cloud cover. The projected efficiency of such a plant would be about 30%, but this fairly low efficiency must be balanced against the facts that energy from the sun costs nothing and that the waste heat from such a plant places virtually no additional burden on the environment. The principal problem with this and other exotic systems for generating electricity is that the time needed for their implementation may be considerable.
3.4 Windmills
Windmills, once widely used for pumping water, have become viable for electric-power generation because of advances in their design and the development of increasingly efficient generators. Windmill “farms,”
at which rows of windmills are joined together as the source of electrical energy, serve as a significant, though minor, source of electrical energy in coastal and plains areas. However, the vagaries of the wind make this a difficult solution to implement on a large scale.
4.0 CONCLUSION
This unit has given you a definition of power supply and has introduced you to some sources of electricity which play a major role or are widely used all over the world. Also the environmental considerations of the sources of power were considered.
5.0 SUMMARY
Power supply can be defined in relation to electrical energy supply as a supply of electrical power (electricity). Though electrical energy occurs naturally, but seldom in forms that can be used therefore has to be converted from other forms of energy to electrical energy and the sources includes amongst many: steam, fossil fuel, solar, windmill, hydro etc.
6.0 TUTOR-MARKED ASSIGNMENT
i. What is meant by power supply?ii. Enumerate the sources of power (electricity).
iii. Give the environmental considerations of the sources of power.
iv. List the merits and demerits of coal as a source of power supply.
7.0 REFERENCES/FURTHER READING
Robert, L. B. (1999). Electronic Devices and Circuit Theory. 7th Edition Prentice-Hall Inc. New Jersey.
Theraja, B. L. and Theraja, A. K. (2010). Electrical Technology. S. C.
Chand, New Delhi, India.
www.wikipedia.com www.howstuffworks.com www.worksheet.com
www.electronicstutorials.com